Data unit sending means and control method

ABSTRACT

The present invention relates to a data unit sending means and a method for controlling a data unit sending means, where data units of a first protocol (L2_ARQ) embed data units of a second protocol (L3) belonging to a higher layer, and said data units of said first protocol (L2_ARQ) are held in a send buffer means. According to the invention, the data units of the second protocol (L3) are discriminated, and the data units of the first protocol (L2_ARQ) that embed a particular data unit of the second protocol (L3) are associated with said particular data unit of the second protocol (L3), and the contents of the send buffer means is managed in accordance with said association.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a data unit sending means and amethod for controlling a data unit sending means.

[0002] In the field of communications, the concept of packet exchange iswell known. A data packet is a given length of data whose structure isdetermined by a given protocol, i.e. a set of rules governing theexchange, manipulation and interpretation of such packets. Depending onthe protocol, different names are used, such as frame, packet, etc. Amore generic term is protocol data unit (PDU), and the followingdescription shall use the term data unit for simplicity.

[0003] The process of sending data via a packet exchange typicallycomprises a plurality of protocols, which are arranged in a hierarchy. Aschematic example of such a hierarchy is shown in FIG. 6. The example ofFIG. 6 shows three layers, a higher layer referred to as L3, a layerbelow L3 referred to as L2_ARQ, and a lowest layer L1. In fact, theterms L3, L2_ARQ and L1 refer to protocols associated with these layers.As an example, L3 can be the internet protocol IP, L2_ARQ may be theradio link protocol RLP known from GSM, and L1 can be any suitablephysical layer protocol.

[0004] In accordance with the concept of layering, data units associatedwith a higher layer are passed to a lower layer, e.g. from L3 to L2-ARQin example of FIG. 6, where the lower layer protocol embeds the higherlayer data units.

[0005] The term “embedding” may refer to encapsulation or segmentation.In the case of encapsulation, a higher layer data unit is placed intoone lower layer data unit, whereas in the case of segmentation, thehigher layer data unit is segmented into smaller pieces of data, eachpiece being placed into a lower layer data unit.

[0006] One of the important aspects of protocol layering is that in adata communication, i.e. in a process where a given amount of data isbeing sent from a source to a destination, the overall path that isassociated with a highest layer comprises sublinks associated with thelower layer protocols, where the endpoints of a protocol of a givenlayer are called peers of said protocol. This concept is well known inthe art and does not need to be described further here. Reference ismade e.g. to the book “TCP/IP, The Protocols” by W. R. Stevens, EdisonWesley 1996.

[0007] [Problem Underlying the Invention]

[0008] Specific problems in connection with the transmission of dataoccur in radio networks, due to the fact that radio links typically havea poorer transmission quality than fixed lines. For the purpose ofexplanation, FIG. 3 shows the architecture for a generic cellularcommunication system. This system consists of a core network (CN) 100,and a part referred to as a radio access network (RAN) 110. The radioaccess network is divided into controller nodes 101 and base transceiverstations (BTS) 102. The hierarchy of the network is such that the corenetwork is connected to several controllers and the controllers areconnected to several base stations. The base stations 102 communicatewith mobile stations (MS) 103.

[0009] A typical problem that will occur when sending data in theup-link direction (i.e. from a mobile station 103 to a base transceiverstation 102) or in the down-link direction (from the base transceiverstation 102 to a mobile station 103) is that errors are introduced overthe radio interface. Such errors are typically due to changes in thetransmission quality, e.g. because the mobile station 103 moves around.Another potential situation for data loss is a handover of acommunication between a given mobile station 103 and a given basetransceiver station 102 to another base transceiver station, when themobile station moves into another cell. Both situations, namely aspecific error condition or a handover lead to the necessity of a linkreset, in the course of which all data in the send buffer of the sendingpeer of the radio link is purged to thereby establish a “clean slate”,such that communication may begin anew in a state unambiguously definedfor both sender and receiver.

[0010] Due to the error characteristics of the radio interface, aso-called ARQ protocol (ARQ=Automatic Repeat reQuest) can optionally beexecuted between the mobile station and the radio access network toreduce the residual error rate. An ARQ protocol comprises the functionof acknowledging the correct receipt of data units by the receivingpeer, where the sending peer implements mechanisms for retransmittingsuch data units that were not correctly received. In this way, thecomplete transmission of data is secured. It may be noted that the useof an ARQ mechanism can be an option associated with a specific mode,i.e. that not every data unit needs to be sent with the ARQ mechanismactivated. As an example, in connection with known protocols there areknown a so-called numbered mode (or I-mode) in which ARQ is activated,and a so-called unnumbered mode, in which no acknowledgment andconsequently no retransmission occurs.

[0011] The first mode is advantageous for data where secure transmissionis a priority, the second mode is advantageous for data where delaysensitivity is a priority and data loss is not so much of a problem,such as real-time voice-over-Internet data.

[0012] In the following, two known types of solutions for securing userdata from being lost in case of a handover of a ARQ protocolcommunication between different network nodes will be described.

[0013] According to a first solution, a protocol state transfer isenacted, i.e. when a handover is performed, the whole state includingstate variables and buffers is moved from the ARQ entity in the RAN(i.e. the peer) to the new network node. Using this mechanism, the ARQentity in the mobile station does not need to know when a handoveroccurs. Such a solution is described e.g. in R. Cohen, B. Patel and A.Segall, “Handover in a Micro-Cell Packet Switched Mobile Network”, ATMJournal of Wireless Networks, Volume 2, no. 1, 1996, pages 13-25, or inS. Powel Ayanoglu, T. F. La Porta, K. K. Sabdani, R. D. Gitlin,“AIRMAIL: A link layer protocol for wireless networks”, ATM/BaltzerWireless Networks Journal, Volume 1, 1995, pages 47-60.

[0014] The benefits of this solution are that no unnecessaryre-transmission of user data over the radio interface occurs, and theARQ protocol in the mobile station can be unaware of the handover, whichmakes the implementation less expensive.

[0015] The disadvantage of this solution is, that it is limited tohandle intra-system handover. This means that both network nodes betweenwhich the handover is executed must operate in accordance with the sameprotocol. If a core network is connected to multiple radio accessnetworks of different type, which do not use exactly the same ARQprotocol, this solution cannot be used, because an inter-system handoveris necessary. Such situations will become more common in the future.

[0016] A different solution for securing user data is that of providingan additional ARQ protocol. In this case, one ARQ protocol is runbetween the mobile station and the radio access network (the basestation controller node) and takes care of errors encountered of theradio interface. The second ARQ protocol is run between the mobilestation and the core network. In case of data loss due to resetting thelink between the mobile station and the base station controller (be itdue to an error condition or a handover), this second ARQ protocol willperform a re-transmission. As an example, in GPRS (General Radio PacketService) the first ARQ protocol is called RLC (Radio Link ControlProtocol) and the second ARQ protocol is called LLC (Link Layer ControlProtocol).

[0017] Although such an arrangement enables the handling of inter-systemhandovers, it has disadvantages. First of all, additional radioresources are consumed due to the overhead introduced by the second ARQprotocol. As an example, in GPRS the overhead per transmitted L3 dataunit introduced by the LLC protocol is in the order of 7 bytes. Comparedto the size of a Van Jacobson compressed TCP acknowledgment, which isunder 10 bytes, the size will almost be doubled when transmitting TCPacknowledgments (in a L3 data unit). Also, the implementation of two ARQprotocols in the mobile station leads to higher costs in terms of memoryand processing power.

OBJECT OF THE INVENTION

[0018] The object of the present invention is to provide a data sendingmeans and a corresponding control method that secure data transmission,but which are applicable to intra-system handovers and inter-systemhandovers without adding unnecessary overhead. It may be noted that thisobject is not restricted to radio networks, because the avoidance ofband-width waste is advantageous in any network. However, radio networksare a preferred application of the invention to be described in thefollowing.

SUMMARY OF INVENTION

[0019] This object is achieved by the subject matter described in theindependent claims. Advantageous embodiments are described in thedependent claims.

[0020] In accordance with the present invention, a data unit sendingmeans that operates in accordance with an ARQ protocol, referred togenerically as L2_ARQ in the following, embeds higher layer data unitsof a protocol L3. L3 data units are discriminated, the L2_ARQ data unitsin which L3 data units are embedded are associated with the L3 dataunits embedded therein, and the contents of the send buffer are managedin accordance with the association.

[0021] The process of handling the data units will be explained in ageneral way in connection with FIG. 1. The L3 data units or PDUs arereceived (step S1) and then discriminated (step S2). Discriminationmeans that individual L3 data units are identified such that one may bediscriminated from the other. It may be noted that this feature departsfrom the conventional approach of making separate protocol layerscompletely transparent to one another, because in the present inventionthe L2_ARQ protocol is made “intelligent” in the sense that it maydiscriminate and differentiate individual L3 data units.

[0022] After the discrimination, the L3 data units are embedded (i.e.encapsulated or segmented) into one or more L2_ARQ data units (step S3).Then, the L2_ARQ data units are brought into association with the higherlayer L3 data units that they form. In other words, a record is kept ofwhich L2_ARQ data units belong to which L3 data unit.

[0023] Then, the L2_ARQ data units are placed into a send buffer, wherethe sending to the receiving L2_ARQ peer is performed in accordance withany suitable or required flow control method. For example, a widely usedflow control method is that of window-based flow control. The precisemethod and its details will be determined by the specific L2_ARQprotocol, such that this is of no importance to the present invention.

[0024] In accordance with the present invention, however, the managementof the send buffer is performed in accordance with the associationbetween the L2_ARQ data units and the higher layer L3 data units thathave been embedded in the L2_ARQ data units. The term management refersto how the contents of the buffer is controlled, i.e. under whichconditions which data units are deleted.

[0025] It may be noted that the order of steps indicated in FIG. 1 isonly an example, and the basic principle of the invention as specifiedby the claims may be implemented in any suitable way.

[0026] By managing the contents of the send buffer, i.e. controlling thedeleting of data units therein in accordance with the associationbetween L2_ARQ data units and the higher layer L3 data units embeddedtherein, it is possible to achieve data transmission security withouttwo layers of ARQ protocols and nonetheless enabling inter-systemhandover. This is due to the fact that by managing the buffer inaccordance with the association of L3 data units and L2_ARQ data units,any loss of L3 data units can be avoided at the L2_ARQ level. As aconsequence, an ARQ mode above the L2_ARQ level is superfluous. At thesame time, as the loss of data is avoided by a mechanism implementedaround the send buffer, an inter-system handover poses no problem.

[0027] It should be noted that the L2_ARQ protocol does not necessarilysend all data units in an ARQ mode. Much rather, the invention isapplicable to any protocol, i.e. also such protocols that providetransmission modes in which no re-transmission occurs. However,according to a preferred embodiment, the buffer management for suchL2_ARQ data units that are sent in an ARQ mode is performed in such away that a given L2_ARQ data unit is only deleted from the send buffermeans if acknowledgments for all L2_ARQ data units associated with thesame L3 unit as said given L2_ARQ data unit have been received. In thisway, all L2_ARQ data units belonging to one L3 data unit are held in thesend buffer until the last L2_ARQ data unit associated with that L3 dataunit has been acknowledged, i.e. until all L2_ARQ data units associatedwith a particular L3 data unit have been acknowledged.

[0028] Then, in the event of a reset, i.e. a resetting of the numberingof L2_ARQ data units, or a handover, the L2_ARQ data units belonging tothe last L3 data unit for which not all L″-ARQ data units have beenacknowledged will simply be sent again. In this way, a part of said L3data unit may be sent twice, but in any case the L3 data unit is sentcompletely, such that no retransmission on the L3 level or higher (e.g.TCP) is necessary. In the event of a handover, the L3 data units in thesend buffer may simply be transferred to the new node, regardless ofsaid new node operating according to the same L2_ARQ protocol or adifferent one. In other words, an inter-system handover is no problem,because no other information besides the buffer contents (in terms of L3data units), such as state variables, etc., must be handed over.

[0029] The present invention will now be described on the basis ofpreferred and detailed embodiments that serve the purpose of explainingthe invention and are not to be seen as restrictive. Reference will bemade to the accompanying figures, in which

[0030]FIG. 1 describes the control of a data unit sending meansaccording to a basic example;

[0031]FIG. 2 is a schematic representation of a data unit sending meansaccording to the invention;

[0032]FIG. 3 shows the architecture of a generic cellular communicationsystem;

[0033]FIG. 4 is an explanatory diagram for describing a detailed exampleof the invention;

[0034]FIG. 5 is an explanatory diagram for explaining the associationbetween L3 data units and L2 data units; and

[0035]FIG. 6 is a representation of a generic protocol stack.

DETAILED DISCLOSURE OF THE INVENTION

[0036]FIG. 2 schematically shows the arrangement of elements forembodying a data unit sending means of the present invention. 1 refersto a discriminator, 2 to an embedder, 3 to a buffer and 4 to a controlmeans. As can be seen, data units from the L3 layer arrive and arediscriminated by discriminator 1 at the L2_ARQ layer. The discriminationresult is passed to the control means 4, that then performs theassociation of L2_ARQ data units generated in the embedder 2 with the L3data units. Then the L2_ARQ data units are passed to the buffer 3, wherethe management of the buffer contents is performed in accordance withthe association between the L2_ARQ data units and the higher layer L3data units that they embed. As already mentioned previously, thespecific flow control performed by the control means 4 will depend onthe specific L2 ARQ protocol.

[0037] As already mentioned in connection with FIG. 1, the structureshown in FIG. 2 is only a schematic example for the purpose ofexplanation, and other arrangements are possible, which the skilledperson will choose as is suitable or desired.

[0038] For the purpose of the following description of detailedembodiments, it will be assumed that the L2_ARQ protocol provides tworeliability modes for the transmission of data units, namely one withacknowledgment and re-transmission, and another without re-transmission.The first mode, which is an ARQ mode, will be referred to as thenumbered mode or I-mode, whereas the second will be referred to as theunnumbered mode or UI-mode. It will also be assumed that the buffer willcontain respective queues associated with the modes. In other words,there will be an I-queue and an UI-queue. Naturally, the presentinvention is by no means restricted to such an arrangement, as there maybe a larger number of reliability modes, such as unreliable,semi-reliable and fully reliable, and there can be a correspondingnumber of queues in the buffer. The semi-reliable or fully reliabletransmission modes can either be combined with in-sequence orout-of-sequence delivery, as will be explained in more detail furtheron.

[0039] The following embodiments will be described on the basis of theprotocol arrangement shown in FIG. 6, where L3 is used to refer to anynetwork protocol, e.g. the internet protocol IP. It may be that the L3data units are embedded in a specific framing scheme, as e.g. providedby the point-to-point protocol PPP. Thus, L3 refers to any protocol thatproduces a corresponding data unit that is transferred to the layerbelow, i.e. the L2_ARQ layer.

[0040] As already discussed above, the L2_ARQ layer refers to a linklayer protocol that embeds L3 data units into L2_ARQ data units, wherethe embedding preferably is a potential segmentation of larger L3 dataunits into smaller L2_ARQ data units and/or concatenation of several L3data units into L2_ARQ data units. As mentioned above, the L2_ARQprotocol implements at least two modes, namely an ARQ mode (I-mode), anda non-acknowledged mode (UI-mode). The basic concept of an ARQ mode suchas the I-mode is well known in the art, such that a further descriptionof details is not necessary. It is sufficient to say that the L2_ARQprotocol contains rules for how the re-transmission of L2_ARQ data unitsis done, on the basis of the presence or absence of acknowledgmentmessages and retransmission requests for specific data units. As isnecessary for any ARQ mechanism, the L2_ARQ data units sent inaccordance with the acknowledgment mode are buffered in the send buffer,where they must at least be held until their correct receipt has beenacknowledged. The preferred buffer management mechanism of the presentinvention shall be described further on.

[0041] For better explaining the invention, reference will be made tothe architecture of a generic communication system as shown in FIG. 4.As can be seen, a mobile station (MS) 500 has an L3 peer thatcommunicates with a corresponding counterpart in the core network (CN)400. The core network 400 is connected to two different radio accessnetworks 401 and 402. Each radio access network comprises a plurality ofradio access network nodes, such as e.g. base station controllers (BSC)and base transceiver stations (BTS), where the schematic representationof FIG. 4 only shows one base transceiver station 403, 404 for each ofthe radio access networks 401 and 402, for the purpose of simplicity.Also, it may be noted that the core network can be connected to morethan two radio access networks.

[0042] There exist two peer entities of the L2_ARQ protocol, one runningin the mobile station 500 and one in the respective radio access network401 or 402. In the example of FIG. 4, each radio access network has itsown L2_ARQ protocol, referred to as L2_ARQ(RAN1) for radio accessnetwork 401 and as L2_ARQ(RAN2) for radio access network 402.

[0043]FIG. 4 also shows a physical layer protocol L1 provided below theL2_ARQ protocol. The L1 peers are directly associated with the physicalconnection, i.e. the transceiver in the mobile station 500 and thetransceiver in the base transceiver station 403 or 404, whereas theL2_ARQ protocol entity on the network side resides in a network node ofthe respective radio access network.

[0044] It will be assumed that a reset of the link between two L2_ARQpeers can occur, e.g. due to a predetermined error condition. Morespecifically, a link reset will lead to the data unit numbering beingreset, to thereby be able to start communication anew. There aredifferent error conditions that may lead to such a resetting of the dataunit numbering, for example if a given data unit has been re-transmitteda predetermined number of times, without an acknowledgment having beenreceived. Another error condition can be that an acknowledgment isreceived for a data unit that was never sent. Both cases indicate thatthe link is strongly disturbed, such that a reset is necessary. Inconventional systems, the data contained in the send buffer is simplypurged in the course of a reset, to thereby give the sending peer andreceiving peer an unambiguously defined starting situation.

[0045] Another potential data loss situation is that of a handover,where a communication is handed from one network node of a radio accessnetwork to another. Such a handover may be either inside of a givenradio access network (intra-system) or may be between two differenttypes of radio-access networks (inter-system). When a handover occurs,this may result in moving the execution point for the L2_ARQ protocolentity to a new physical node where a L2_ARQ entity will be started andcontinue the communication with a new L2_ARQ peer. As can be seen inFIG. 4, this means that in an inter-system handover, the communicationnot only needs to be handed over from e.g. base station 403 to basestation 404, but a handover will also occur inside of the mobile station500, as the transmission must be handed from L2_ARQ (RAN1) to L2_ARQ(RAN2). In any case, a handover will include a link reset, as the twopeers of the communication require a defined starting point.

[0046] Now the preferred buffer management of the send buffer for asending peer (be it in the mobile station, or be it in a network node)will be described. The sender of both L2_ARQ entities in a system isonly allowed to delete L2_ARQ data units from its send buffer when allL2_ARQ data units associated with a whole L3 data unit have beenacknowledged by the peer entity. If concatenation of L3 data units isused, a L2_ARQ data unit may not be deleted if it includes a segmentedL3 data unit that was not fully received. Further, if the peer L2_ARQreceiver uses in-sequence delivery, the L2-ARQ sender is only allowed toremove the L2_ARQ data units using in-sequence removal.

[0047] This basic principle will be explained in more detail inconnection with FIG. 5. FIG. 5 shows a simple example for thesegmentation/concatenation of L3 data units into L2_ARQ data units. AnL2 ARQ protocol entity has segmented three L3 data units, denotedL1#1-L3# into five L2_ARQ data units, denoted L2#1-L2#5. As may be seen,L3#1 is segmented into L2#1-L2#3, L3#2 is segmented into L2#3-L2#4, andfinally L3#3 is completely enclosed in L2#5.

[0048] The following table 1 shows examples of the contents of theL2_ARQ send buffer when different data units have been acknowledged bythe peer entity. The sender works according to the above-describedmechanism, i.e. L2_ARQ data units are only deleted if complete L3 dataunits have been acknowledged. Table 1 indicates the differences forin-sequence and out-of-sequence delivery. Acknowledged PDUs by peer(L2#x) Send buffer contents In- Out-of- (L2#x) sequence sequence 1 2 3 45 delivery delivery x x x x 1-5 1-4 x x x x 3-5 3-4 x x x x 5 5 x x x1-5 1-3 x x 1-5 1-3, 5 x 1-5 1-4 x x x x 5 5 x x x x 1-5 1-3

[0049] As can be seen in the first line, in the case of in-sequencedelivery all L2_ARQ data units remain in the send buffer, because thefirst L3 data unit L3#1 has not been completely acknowledged becauseL2#3 was not acknowledged. In the case of out-of-sequence delivery, thedata unit L2#5 has been deleted, as the acknowledgment of L2#5 meansthat L3#3 has been completely acknowledged. In the second line, it canbe seen that the acknowledgment of L2#1 to L2#3 means that L3#1 has beencompletely acknowledged, such that L2#1 and L2#2 may be deleted, but dueto the fact that L2#4 has not been acknowledged, L2#3 may not be deletedas L2#3 also is associated with L3#2.

[0050] In the third line, L2#1 to L2#4 were acknowledged, i.e. L3#1 andL3#2. Consequently, only L2#5 remains in the send buffer. In the case ofline 4, L2#3 to L2#5 have been acknowledged, such that L3#2 and L3#3have been acknowledged, so that for out-of-sequence delivery only L2#1to L2#3 remain in the buffer, whereas for in-sequence delivery allL2_ARQ data units remain, because the first unit was not acknowledged.The remaining examples are self-explanatory.

[0051] In a general sense, any time during the protocol operation, theL2_ARQ entity must be able to provide information about the contents ofits send buffer in terms of the L3 data units. This information can bethe identities of the L3 data units in accordance with any suitableaddressing scheme, or the L3 data units themselves, where these L3 dataunits are those for which the associated L2_ARQ data units have not beenfully acknowledged by the peer entity.

[0052] Now an example will be described, in which a link reset occurs,i.e. the resetting of the data unit numbering, without a handover. Inother words, the sending and receiving peers remain the same, but thenumbering of the I-mode data units is reset, e.g. due to a given errorcondition. In this case, the sending peer will simply renumber theL2_ARQ data units in its send buffer in such a way that the first L2_ARQdata unit of the new sequence is the first L2_ARQ data unit associatedwith the last L3 data unit that was not completely acknowledged. Inother words, when considering the example shown in FIG. 5, if oneassumes that L2#1 to L2#3 have been acknowledged, which means that L3#1has been acknowledged, the new sequence will begin with L2#3 as itsfirst data unit, because L3#2 was not fully acknowledged prior to thereset. In this way there is no possibility of data loss in the course ofthe reset.

[0053] In the example just described, the segmentation performed priorto the link reset was retained. Preferably, the link reset will beperformed by additionally resegmenting the L3 data units for which notall L2_ARQ data units were acknowledged. In other words, when taking theabove assumption that L2#1 to L2#3 were acknowledged, L3#2 and L3#3would be resegmented such that the first L2_ARQ data unit of the resetsequence would only be associated with L3#2. As can be seen, this hasthe advantage that the end part of L3#1 is not unnecessarilyretransmitted, i.e. in a general sense that there is no unnecessaryretransmission of data.

[0054] It may be noted that for the above examples, it makes nodifference if the sending peer is in the mobile station or the networknode.

[0055] Now the situation of a handover will be described.

[0056] According to one embodiment, when a handover occurs, whichresults in moving the L2_ARQ protocol execution to a new physical node,the old L2_ARQ entity will be terminated and a new L2_ARQ entity will bestarted. Before the old entity is terminated, the contents of the sendbuffer, i.e. the unacknowledged L3 data units, will be transferred tothe newly established L2_ARQ entity. The new entity will then resume thetransmission starting with the unacknowledged L3 data units receivedfrom the terminated L2_ARQ entity.

[0057] In other words, in accordance with the present invention, the newL2_ARQ entity will start sending the first L2_ARQ data unit associatedwith the last L3 data unit that was not completely acknowledged prior totermination of the old L2_ARQ entity. In this way, similar to the aboveexample of a reset without a handover, the complete transmission of allL3 data units is secured, without the necessity of running an ARQ modeon any higher level to protect against such losses. Also, due to thefact that L3 data units are passed on, an inter-system handover is noproblem. In other words, no state variables need to be passed on, andthe new node can handle the L3 data units in its own particular way,i.e. according to its particular L2_ARQ protocol. This especially meansthat the L3 data units may for example be resegmented differently in thenew node.

[0058] The mechanisms with which unacknowledged L3 data units aretransferred between the old and the new L2_ARQ entity may be selected inany suitable or desirable way in accordance with the specific hardwareand protocols involved. In the network, the transfer of unacknowledgedL3 data units can be done e.g. by “pushing back” the data units to thecore network, which then takes care of the data unit delivery/transferto the new L2_ARQ entity. This can be very useful in case of aninter-system handover, where the different radio access networks are notconnected to each other. An alternative is that the old network nodewill directly transfer the L3 data units to the new network node, ifthis new node is known. This alternative is advantageous in anintra-system handover.

[0059] When the data unit sender is in the mobile station, two differenthandover procedures can be distinguished for the L2_ARQ entity. In caseof an intra-system handover, the L2_ARQ entity will only have to performa reset. This may involve the renumbering of already segmented L2_ARQdata units, or the resegmentation and/or reconcatanation of the sendbuffer contents, after which the protocol is restarted. When aninter-system handover is performed, the unacknowledged L3 data units mayhave to be transferred to a new L2_ARQ entity. This entity may beexecuted in the same physical device (mechanical/electronic piece, chip,CPU, etc.) or in a different physical device located in the same mobilestation. The precise details of such a transfer from e.g. L2_ARQ (RAN1)to L2_ARQ (RAN2) of mobile station 500 in FIG. 4 will depend on theprecise nature of the radio access networks, the employed protocols, thehardware of the radio network and mobile station, etc. It is clear thatthis can be done in any suitable or desirable way under the specificconditions of the situation.

[0060] According to another embodiment, the handover is not performed bya direct transfer of the L3 data units left in the send buffer, muchrather a multicast group is formed. More specifically, a set of at leasttwo L2_ARQ entities in the network is created, that forms a multicastgroup, e.g. an IP multicast group. Only one of the L2_ARQ entities fromthe group at a time communicates with the peer entity in the mobilestation, and this L2_ARQ entity will be referred to as the “serving”L2_ARQ entity. The rest of the L2_ARQ entities in the multicast group donot have a peer-to-peer communication with the mobile station, and willbe denoted as the “passive” L2_ARQ entities.

[0061] The members of the multicast group can be chosen in any suitableor desirable way. Preferably, in a cellular mobile communication system,the group includes all nodes that are potential handover candidates,i.e. those nodes associated with the cells adjacent to the cell in whichthe mobile station is presently located. Naturally, this is only anexample, and other criteria are possible for determining the members ofthe group.

[0062] It may be noted that the multicast group consisting of L2_ARQentities is not a fixed group, but may be adapted in accordance with howthe mobile station moves about. For example, when the mobile stationmoves from one cell to another, the passive members of the multicastgroup are changed to those cells lying around the new cell. In otherwords, when the mobile station moves around in the network, a passivemember of the group can be deleted and new ones may be added to thegroup.

[0063] There are different possibilities of using the multicast group toperform a handover. According to one alternative, the L2_ARQ peers workin the same way as previously described, i.e. those L3 data units areretained for which not all associated L2_ARQ data units have beenacknowledged. Then, when the handover is performed, the unacknowledgedL3 data units will be multicast to the whole group and the new servingL2_ARQ entity will resume the transmission starting with the multicastdata units, i.e. the first data unit will be associated with the last L3data unit for which not all associated L2_ARQ data units wereacknowledged prior to the handover. Although this solution entails thetransmitting of a larger amount of data than in the previous embodiment,it has the advantage that the serving node does not have to know towhich node the L2_ARQ communication is handed over.

[0064] According to another alternative, the L2_ARQ peers again performthe buffer management as described above, until a condition occurs thatindicates that a handover might have to be performed. Then, acorresponding control process predicting the handover can trigger thestart of a multicast session. When the start is triggered, the L2_ARQbegins by multicasting the unacknowledged L3 data units currentlyresiding in its send buffer. The whole multicast group will then receivethe new L3 data units. Either the core network or the serving L2_ARQentity will be responsible for this transmission. The serving L2_ARQentity will then in addition continue to regularly send multicastmessages to inform the passive L2_ARQ entities which L3 data units canbe discarded from the send buffers. That is, information about the L3data units that have been acknowledged by the peer L2_ARQ entity ismulticast. When a handover occurs, the serving L2_ARQ entity willmulticast a data unit discard message if new L3 data units have beenacknowledged since the last discard message was sent. Alternatively, theserving L2_ARQ that is handing over the communication can transmit amulticast message that indicates with which L3 data unit to continuesending after handover.,

[0065] In order to accomplish the above system, some kind of addressingscheme must be employed for the L3 data units. Any suitable or desirablescheme that uniquely identifies the L3 data units, and in the case ofin-sequence delivery of L3 data units identifies the internal order, maybe used.

[0066] One example for an addressing scheme is the use of sequencenumbering for each L3 data unit. The precise implementation of such ascheme, e.g. the nature of a common protocol between the members of themulticast group, is of no relevance for the present invention, as longas the above described function is given.

[0067] Up to now, the buffer management and handling of L2_ARQ dataunits in an acknowledgment mode (the I-mode) were discussed. Themanagement of the L2_ARQ data units of the non-acknowledgment mode(UI-mode) in the send buffer is somewhat different, but the handlingafter reset or after handover is basically the same. More specifically,the L2_ARQ data units that are in the UI-queue are simply deleted ordiscarded after having been sent. In this sense, the contents of theUI-queue in the send buffer always reflects those data units that havenot yet been sent. When a reset or a handover occurs, the new L2_ARQentity will simply continue the procedure by sending those L2_ARQ dataunits of the UI-mode that have not yet been sent. The process ofbringing the L3 data units that have not yet been sent from the oldL2_ARQ entity to the new L2_ARQ entity during a handover is exactly asdescribed for the I-mode L2_ARQ data units. In other words, this can bedone by direct transfer, by pushing back through the core network, or inthe way of the above-described multicast group.

[0068] According to a preferred embodiment, the UI-mode data units arehowever treated somewhat differently, namely in the event of a reset ora handover, all those L2_ARQ data units that are associated with a L3data unit for which only a part of the L2_ARQ data units was sent, aredeleted prior to recommencing sending after the reset or the handover.

[0069] This has the advantage that an unnecessary transmission of L2_ARQdata units can be avoided. More specifically, one functionality of theL2_ARQ protocol will be that of error detection, which means performingan error check and discarding L3 data units that have incurredtransmission errors. Typically, in the course of a reset or a handover,where parts of a given L3 data unit have been sent, said given L3 dataunit will be incomplete and therefore discarded. As a consequence, thesending of the remaining L2_ARQ data units associated with said given L3data unit is superfluous.

[0070] As already mentioned above, one of the advantages of the presentinvention is that no ARQ mode needs to be run above the L2_ARQ level.However, it is preferred that the L2_ARQ protocol implements some sortof error check and error control. For example, the receipt of faulty L3data units should be recognized, and these faulty data units should bediscarded. In this way, the system of the present invention becomesparticularly effective, because then those I-mode L2_ARQ data units thatonly form part of an L3 data unit are discarded, and due to the buffermanagement of the present invention, the complete L3 data unit is sentthereafter (see above description of reset and handover with respect toI-mode L2_ARQ data units), such that the complete transmission of L3data units is secured.

[0071] Consequently, the L2_ARQ implementation will not only be able toperform error detection on the L2_ARQ level (e.g. as will typically beassociated with the acknowledgment mode for L2_ARQ data units), but willalso be able to perform error detection on the L3 level.

[0072] One of the important advantages of the present invention is thatit only requires the modification of a data unit sending means. Thisgreatly simplifies the implementation of the present invention intoexisting systems.

[0073] Reference signs in the claims are intended for a betterunderstanding and do not restrict the scope.

1. A method for controlling a data unit sender, said data unit senderhaving a send buffer for holding data units to be sent and said dataunit ender being arranged to send data units of a first protocol toanother peer of said first protocol, and embed data units of a secondprotocol belonging to a higher layer than said first protocol into dataunits of said first protocol, said method comprising the steps:discriminating said data units of said second protocol, associating dataunits of said first protocol with data units of said second protocol,managing the contents of said send buffer means in accordance with theassociation between the data units of said first protocol and saidsecond protocol.
 2. The method according to claim 1, wherein said firstprotocol provides at least one acknowledgment mode in which theacknowledgment of the correct receipt of data units sent to said otherpeer is provided, and said managing comprises deleting a given data unitof said first protocol that belongs to said acknowledgment mode fromsaid send buffer means only if acknowledgments for all data units ofsaid first protocol associated with the same data unit of said secondprotocol as said given data unit of said first protocol, have beenreceived.
 3. (CANCELED)
 4. (CANCELED)
 5. The method according to claim1, wherein said data unit sender is a mobile station or a radio networknode in a mobile communication network, and said first protocol controlsa radio link.
 6. (CANCELED)
 7. (CANCELED)
 8. (CANCELED)
 9. (CANCELED)10. (CANCELED)
 11. (CANCELED)
 12. (CANCELED)
 13. (CANCELED) 14.(CANCELED)
 15. (CANCELED)
 16. (CANCELED)
 17. (CANCELED)
 18. A data unitsending means having a send buffer means for holding data units to besent and said data unit sending means being arranged to send data unitsof a first protocol to another peer of said first protocol, and havingan embedding means for embedding data units of a second protocolbelonging to a higher layer than said first protocol into data units ofsaid first protocol, a discriminating means for discriminating said dataunits of said second protocol, an associating means for associating dataunits of said first protocol with data units of said second protocol, acontrol means for managing the contents of said send buffer means inaccordance with the association between the data units of said firstprotocol and said second protocol.
 19. The data unit sender according toclaim 26, wherein said first protocol provides at least oneacknowledgment mode in which the acknowledgment of the correct receiptof data units sent to said other peer is provided, and said controllerfor managing the buffer is arranged to delete a given data unit of saidfirst protocol that belongs to said acknowledgment mode from said sendbuffer only if acknowledgments for all data units of said first protocolassociated with the same data unit of said second protocol as said givendata unit of said first protocol, have been received.
 20. (CANCELED) 21.(CANCELED)
 22. The data unit sender according to claim 26, wherein saiddata unit sender is a mobile station or a radio network node in a mobilecommunication network, and said first protocol controls a radio link.23. (CANCELED)
 24. (CANCELED)
 25. (CANCELED)
 26. A data unit senderhaving a send buffer for holding data units to be sent and said dataunit sender being arranged to send data units of a first protocol toanother peer of said first protocol, and having an embedder forembedding data units of a second protocol belonging to a higher layerthan said first protocol into data units of said first protocol, adiscriminator for discriminating said data units of said secondprotocol, an associator for associating data units of said firstprotocol with data units of said second protocol, a controller formanaging the contents of said send buffer in accordance with theassociation between the data units of said first protocol and saidsecond protocol (U).